| Poxviruses are renowned for encoding proteins that modulate virtually every aspect of the host immune system. One effective barrier against virus infection is apoptosis, a form of programmed cell death. Apoptosis is controlled at the mitochondria by pro- and anti-apoptotic members of the highly conserved Bcl-2 family of proteins, and two members in particular, Bak and Bax, are absolutely critical to the induction of cell death. Although poxviruses encode an array of effective inhibitors of apoptosis, only members of the Avipoxvirus genus, of which fowlpox virus is the prototypical member, encode proteins with obvious, albeit limited, sequence identity to cellular Bcl-2 proteins. Fowlpox virus, the prototypical avipoxvirus, encodes FPV039, a protein that possesses two of the four highly conserved Bcl-2 homology (BH) domains that characterise the Bcl-2 family. Here we demonstrate that, like cellular Bcl-2 proteins, FPV039 localised to the mitochondria where it prevented apoptosis induced by a variety of cytotoxic stimuli, including virus infection itself. FPV039 inhibited apoptosis induced by Bak and Bax through an interaction with Bak and activated Bax. FPV039 also interacted with a discrete subset of BH3-only proteins, the upstream activators of Bak and Bax, to prevent Bax activation in the first place. Additionally, we have characterised the function and mechanism of action of a novel deerpox virus protein, DPV022. Intriguingly, DPV022 lacks obvious homology to cellular Bcl-2 proteins but shares limited regions of amino acid identity with two other poxviral inhibitors of apoptosis, vaccinia virus F1L and myxoma virus M11L, which are themselves unrelated. Here we demonstrate that DPV022 localised to the mitochondria where it interacted directly with Bak and Bax to inhibit apoptosis, even in the absence all cellular anti-apoptotic Bcl-2 proteins. We have also embarked on a preliminary analysis of the apical events that initially trigger apoptosis during infection with vaccinia virus, the prototypical poxvirus. Accordingly, we demonstrate that the BH3-only protein Noxa contributed to the vaccinia virus-induced apoptotic response, possibly through an involvement with dsRNA. Together, this study represents a comprehensive analysis of the ways in which poxviruses manipulate the cellular Bcl-2 family of proteins, the arbiters of cell death. |
